Organometallic precursor processing has been used in recent years to prepare advanced materials such as titanium nitride, silicon carbide, silicon nitride, and boron nitride. This method has several advantages over classical techniques, e.g., relatively low temperature processing requirements, ease of control for maintaining high purity, and formability of the ultimately produced advanced material into fibers, coatings, films, etc. Generally, the organometallic precursors are transformed into the corresponding advanced materials by a pyrolytic process.
Titanium nitride is a particularly useful advanced material having several desirable properties such as, for example, a high melting point (2,950.degree. C.), high hardness (8-9 on the Moh scale), excellent strength (34,000 psi bending strength, and 141,000 psi compression strength), high thermal conductivity, and nonreactivity with a variety of other materials. Furthermore, it is unaffected by acids, excepting aqua regia; however, alkali compounds may cause its decomposition.
The conventional methods for preparing titanium nitride involve the high temperature reaction of a source of titanium such as, for example, titanium tetrachloride or titanium metal, with a source of nitrogen such as, for example, ammonia or nitrogen.
It is known to prepare titanium nitride by the pyrolysis of a polymeric precursor formed by reacting ammonia with a titanium dialkylamide. See Brown, G. M. and Maya, L., "Ammonolysis Products of the Dialkylamides of Titanium, Zirconium, and Niobium as Precursors to Metal Nitrides," Journal of the American Ceramic Society, v. 71 (1988) 78-82. Specifically, a titanium dialkylamide such as, for example, tetrakis(dimethylamido)titanium is reacted with liquid anhydrous ammonia to form an imido- or nitrido-bridged polymeric precursor having the general formula Ti.sub.2 (NX.sub.2)(NH.sub.2) N.sub.3, wherein X is an alkyl group. Thereafter, the precursor is pyrolyzed in an ammonia atmosphere to prepare titanium nitride. During the initial stages of the pyrolysis process, NHX.sub.2 and NH.sub.3 are released, forming a compound having the approximate composition Ti.sub.3 N.sub.4. At a temperature of approximately 700.degree. C. to 800.degree. C., additional nitrogen is released thereby forming partially crystalline titanium nitride.
Also, it is known to prepare titanium nitride by the pyrolysis of a polymeric precursor formed by reacting tetrakis(dimethylamido)titanium with bifunctional amines. See Seyferth, D. and Miganani, G , "The Preparation of Titanium Nitride and Titanium Carbonitride by the Preceramic Polymer Route," Gov. Rep. Announce. Index (US), v. 88 (1988) 827, 109. The publication discloses that tetrakis(dimethylamido)titanium is reacted with a diamine such as, for example, CH.sub.3 NHCH.sub.2 CH.sub.2 NHCH.sub.3 to form a polymeric precursor which pyrolyzes under a stream of ammonia to give fairly pure titanium nitride. Pyrolysis is carried out at a temperature of approximately 800.degree. C. to 1,200.degree. C. to form amorphous titanium nitride, which must thereafter be calcined at approximately 1,500.degree. C. to form crystalline titanium nitride.
In light of the prior art, there is recognized a need for developing a process employing organometallic precursors which are pyrolyzed at lower temperatures to prepare crystalline titanium nitride.
In Bruger, H. and Wannagat, U., "Uber Titan--Stickstoff-Verbindungen, 2.Mitt.," Mh. Chem., Bd. 4 (1963) 761, a process is disclosed for reacting titanium tetrachloride with N,N-bis(trimethylsilyl)amine to produce an adduct, TiCl.sub.4 .multidot.HN[Si(CH.sub.3).sub.2 .multidot.].sub.2. The publication does not teach nor suggest that the organometallic compound would be useful for preparing titanium nitride by a pyrolytic process.
Likewise, Andrianov, K. A., Astakhin, V. V., Kochkin, D. A., and Sukhanova, I. V., "Reaction of Hexamethyldisilazane With Aluminum and Titanium Halides Method of Synthesizing Trialkylhalosilanes," Zh. Obsh. Khim., v. 31, n. 10 (1961) 3,410 discloses a method for Preparing trimethlyhalosilanes by reacting together an equimolar mixture of hexamethyldisilazane and a titanium halide such as, for example, titanium tetrachloride. The authors describe the organometallic reaction byproduct as being NHTiCl.sub.2, but do not suggest that the material could be used to form titanium nitride by pyrolysis.
Finally, U.S. Pat. No. 4,482,689 to Haluska discloses a process for preparing polymetallo(disily)silazane polymers, which are useful in the preparation of ceramic materials by pyrolysis in an inert atmosphere or in a vacuum. Specifically, a chlorine-containing disilane, a disilazane such as, for example, hexamethyldisilazane, and a metal halide such as, for example, titanium tetrachloride, are reacted together in an inert, essentially anhydrous atmosphere to produce a trialkylsilylamino-containing metallosilazane polymer, which may thereafter be pyrolyzed at a temperature between 750.degree. C. and 1,200.degree. C. to prepare an amorphous ceramic material containing titanium nitride.
It would be desirable to prepare organometallic precursors, by a simple process, which precursors could be pyrolyzed at lower temperatures than presently disclosed in the prior art, to produce crystalline titanium nitride.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention, nor that the cited prior art when considered in combination suggests the present invention absent the teachings herein.